The present invention relates to electromechanical actuators having non-dynamic or pseudo-static driving mechanisms and the control and driving of such actuators.
The small motor market has been increasing continuously for decades and there is a particular interest in high performance miniature motors that can be fabricated at low costs. Force and precision are the typical main properties of importance. Common electric motors have in some applications reached their limits and alternatives are being developed. This invention relates to the need for high performance miniature motors.
Electromechanical motors, comprising e.g. piezoelectric motors, is a more and more used type of miniature motors. Piezoelectric actuators are today well known and used in a wide variety of applications. Piezoelectric actuators are generally characterised by a high momentum but a small stroke. By repeating the motion with a high frequency, macroscopic strokes can be achieved. There are a number of fundamentally different operation mechanisms used in electromechanical motors. By using the inertia of some motor component and/or time dependent physical effects, various electromechanical motors can be realized. This group of driving mechanisms may be referred to as dynamic driving mechanisms. Typically, motors with dynamic driving mechanisms can only operate in a certain frequency range, while no operation at low internal speed or frequency is possible. The commonly encountered names ultrasonic and travelling wave motors belong to this group of dynamic driving mechanisms.
Another group of driving mechanisms can be denoted as non-dynamic, static or quasi-static. These non-dynamic mechanisms are characterised in that the motion can be made at arbitrarily low frequencies or speed of the active elements. The driven component is moved by actuator elements which typically make motion cycles such as grip, move, release and return. When one set of elements is releasing, another set of elements will grip the driven component. Typically, the non-dynamic mechanisms are advantageous where controlled positioning is desired at low to medium-high speeds. Further this mechanism allows for easy optimisation in various applications and gives the possibility to deliver high forces. The main disadvantage is the high demands on the construction in order to achieve the desired mechanism. Various solutions to simplify the constructions without losses in performance are therefore generally of great commercial interest.
One mechanisms for non-dynamic motion is the xe2x80x9cinchwormxe2x80x9d mechanism, first disclosed in the patent U.S. Pat. No. 3,902,084. The driven component is moved by mechanical steps in a clamp-extend-unclamp fashion, e.g. in U.S. Pat. No. 5,751,090. There has to be at least two sets of clamping elements that move out of phase. In between each motion, the extention, the driven components is clamped by both sets of elements and stands still. The motion is cyclic and the ultimate resolution corresponds to one step length divided by the voltage resolution. The driven component can in some cases be stopped at fractions of the full step length, a kind of micro-step mode. The clamping and unclamping takes place during a non-motion phase.
In the international patent application WO 97/36366 a piezoelectric motor based on a non-dynamic driving mechanism is disclosed. The mechanism is an alternative to the xe2x80x9cinch-wormxe2x80x9d mechanism and could be denominated a xe2x80x9cmechanical stepping mechanismxe2x80x9d. The motor is made of an electromechanical material as a monolithic multilayer unit with at least two independent sets of drive elements that can move two-dimensionally. The motion of each set is characterised by the four sequences of gripping, moving, releasing and returning. Voltages cycles are applied to the sets of bimorph drive elements, which are out of phase with each other. In the application the preferred voltage cycles were stated to be sinusoidal. Such drive cycles result in elliptical motion trajectories of the contact points of the drive elements. With two element sets phase shifted 180 degrees, the tangential speed of the elements at the instant when one set is gripping and the other is releasing is essentially zero.
Prior art non-dynamically driven electromechanical motors exhibit large advantages. However, some minor disadvantages are still present. One is the problem of achieving controlled stepping with extremely small step lengths, while the quasi-static control of driving still is maintained. Another general problem is that the tangential speed varies constantly during the drive cycle which sometimes results in problems such as wear and vibrations. Further, in a similar way, the normal motion will also vary which apart from creating vibrations and affecting the friction also results in unnecessary power consumption. While the tangential motion can be used for useful work, the normal motion is only necessary to overcome surface irregularities etc. and is not used for power output. A particular problem with the disclosed xe2x80x9cmechanical stepping mechanismxe2x80x9d device is that the sinusoidal voltage cycles do not make use of the full possible stroke of the elements, which lowers the efficiency.
In ultrasonic motors the wear of the contact surfaces is a non-negligible problem. Several solutions to the problems have been suggested including polymer surface and lubrication. The wear of a non-resonant motor is less due to the more controlled motion of the drive elements. However, when high performance nature motors are considered, also a minor wear might affect the performance.
There are numerous ways to make piezoceramic motors according to the present invention but with prior art solutions it is difficult to achieve small size, high forces and low price at the same time.
A general object of the present invention is to provide an improved electromechanical motor, having a non-dynamic or pseudo-static driving mechanism. A further object is to utilise the geometrical motion possibilities of the driving elements in an optimal manner. Another object is to provide a motor with an even motion at constant speed as well as at acceleration or retardation. Yet another object with the present invention is to improve the positioning precision of electromechanical motors, in particular to achieve extremely small step lengths that easily can be used in closed-loop applications with a suitable position sensor. It is also an object to reduce the power requirements as well as improving the precision and force in relation to the volume of the active elements. Yet a further object is to provide an electromechanical motor with improved geometrical and material selections as well as improved frictional properties. Also improved guiding of the driven body and the application of normal force is an object.
The above objects are achieved by devices and methods according to the enclosed claims. In general words, the invention provides a method for driving an electromechanical actuator arrangement having a plurality of drive elements according to a waling mechanism in a motion relative a body. A driving portion of the drive elements are independent movable along and perpendicular to the surface of said body, i.e. in a tangential and normal direction, respectively. According to the method the drive elements are driver in the fours cycle sequences; gripping the body, moving the body, releasing the body and returning to the original position. The drive elements are divided into at least two exclusive sets. The driving of the second set is phase shifted related to the driving of the first set. At least one of the sets is in contact with the body surface at each instant. The method according to the present invention is characterised in that the driving of said first and second set of drive elements during the gripping sequence is performed in such a way that the drive elements have a velocity which has a significant component in the main motion direction. In other words, during the gripping, the driving portion moves with a tangential velocity component.
In preferred embodiments, the driving of the drive elements is performed to make an overlap between the gripping and releasing sequences, respectively, of the two sets. The motion and in particular the tangential velocity component of the drive elements are preferably performed according to a application specific velocity schedule, in particular during the gripping, moving and releasing sequences. One embodiment has a pure tangential motion during the moving sequence. According to one embodiment of the invention, the motion cycle is possible to divide into microsteps, in order to be able to halt the motion at any position. A preferred embodiment also enables a variable step length of the driving cycle.
According to another aspect of the present invention, an electromechanical actuator is presented, which is possible to drive according to the above method. Furthermore, preferred embodiments comprise active portions and connecting passive portions of a monolithic body. The passive portion comprises additional electrode layers, which either are connected to ground, for improving thermal conductivity. Furthermore, the driving portions are made by a material with high thermal conductivity. The sets are also substantially symmetrical with respect to the centre of said electromechanical actuator arrangement.
The present invention introduces several advantages, compared with prior art. The geometrical dimensions are utilised more efficient. The motion of the body is possible to make smooth. The precision of the positioning is enhanced. Furthermore, the energy consumption is reduced.